This invention relates to a method for making Schottky barrier semiconductor diodes and, more particularly, to low noise gallium arsenide devices suitable for use as microwave or millimeter wave frequency mixers.
Schottky barrier diodes are devices in which a rectifying junction is formed by a metal contact on a semiconductor surface. Such devices, particularly when the semiconductor is gallium arsenide, can be switched between conductive and nonconductive states at extremely high repetition rates. Thus, they are attractive for use as high frequency mixers. At high frequencies, however, it is of paramount importance that such parameters as junction capacitance, reverse bias breakdown voltage, and noise be reproducible and highly predictable.
The electrical parameters of Schottky barrier diodes are extremely sensitive to the cleanliness of the metal-semiconductor interface. In particular, contaminants can act as surface traps which degrade the ideal current-voltage characteristics of the device and also result in hysteresis effects. The surface traps also increase the diode noise. Thin layers of native oxides at the metal-semiconductor interface also tend to increase the series resistance of the junction, thus, decreasing the cutoff frequency of the device.
Greater control and reproducibility of the active semiconductor regions has been achieved by use of epitaxial growth to form the regions and by use of ion implantation to control the carrier concentration. With most epitaxial techniques, however, there has been difficulty in controllably producing thin layers and desired doping profiles. Because of outdiffusion, there has also been difficulty in depositing an epitaxial layer on a heavily doped substrate with an abrupt change in carrier concentration between the substrate and the epitaxial layer.
Another problem in making reproducible Schottky barrier diodes is the tendency for preferential reverse bias breakdown to occur at the edges of the metal contact. For this reason, guard rings and other complex structures have been used to prevent current from flowing across the edge portion of the barrier under reverse bias conditions. The edge effects are a result of an increase in the electric field concentration at the periphery of the junction. Typically, the guard rings are of the metal-insulator-semiconductor or the p-n type. Thus, either an insulator or a region of opposite conductivity type circumscribes the edge of the junction. Unfortunately, the use of a guard ring introduces parasitic capacitance and hence decreases the cutoff frequency.